TBILISI, Georgia (CNN) -- Russia has likely moved additional troops into Georgia and its breakaway provinces over the past several days, several administration officials told CNN on Wednesday.

The officials said Russia probably has at least 15,000 troops in the region. The U.S. government had estimated 8,000 to 10,000 Russian troops had moved into Georgia when fighting began last week. A Bush administration official stressed that the scope of Russia's military effort remains unclear.

Russia was blasted diplomatically Wednesday by President Bush over reports of ongoing Russian military operations.

U.S. Secretary of State Condoleezza Rice also said Russian military action in Georgia "must stop and must stop now."

Georgia and Russia accused each other of violating the cease-fire deal agreed Tuesday to end fighting over the breakaway South Ossetia province.

Bush said he was told the Russian military had blocked Georgia's major east-west highway and had soldiers at the main port at Poti, and there were reports that some ships had been attacked.

Russian Foreign Minister Sergei Lavrov said that there were no Russian troops in Poti but that there were soldiers on the outskirts of Gori and Senaki, to the west, according to the Russian news agency Interfax. Senaki is south of another separatist province, Abkhazia.

"They are staying there to neutralize the large arsenal of weapons and other military hardware in the areas of Gori and Senaki. These arsenals remain unguarded. Apparently those who guarded them fled," Lavrov said.

He added that Russia's operations are "aimed at ensuring the peace-enforcement operation in respect to the Georgian side, which violates all of its obligations," and his office denied that Russia had violated the cease-fire.

International agreements signed in the early 1990s allow Russian peacekeepers to maintain a presence in South Ossetia and Abkhazia.

Bush said he was sending Rice to France and Georgia to discuss the violence over disputed provinces within Georgia's borders and to express "America's unwavering support" to the Georgian government. Watch Bush pledge "unwavering support" for Georgia »

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He also announced U.S. aircraft and ships would deliver humanitarian aid to victims of the fighting between Russian and Georgian troops, which erupted Friday.

A U.S. Air Force C-17 cargo jet carrying medical supplies already has arrived in Georgia's capital, Tbilisi, with more aid missions are planned by the Navy and Air Force.

Georgia President Mikheil Saakashvili said Russian forces have been "advancing slowly but surely towards the capital" of Georgia in violation of the cease-fire agreement. He accused the Russians of never intending to hold up their end of the truce.

"The cease-fire never stood in the first place," Saakashvili told CNN's "Situation Room" on Wednesday. "Russia never intended to stop fire."

He added that "Russian tanks continue to ravage Georgia town and villages, killing people, destroying buildings, looting. ... They've been doing worse things to what I've heard in the past and I could never imagine happening in my country."

A spokesman for Russia's Foreign Ministry bristled at reports of Russian troop movements and said they "are not reflecting the real situation." Interactive map: See how far the Russians advanced »

"We as the Russian Federation are [sticking] to the agreement, which [has] been made in Moscow yesterday, and we hope that the other side will show its readiness to do the same," Andrei Nesterenko said.

He chided Saakashvili for "blaming again us for doing something which is contrary to the real situation."

A Russian convoy was seen by a CNN correspondent as it traveled on the road from Gori eastward toward the Georgia capital, Tbilisi -- well outside the mandated area for Russian peacekeepers confined to South Ossetia. Watch Russian spokesman explain tank movement

However, not far from Tbilisi, the convoy turned north into territory between Gori and South Ossetia, CNN correspondent Matthew Chance said.

A Russian government spokesman said the soldiers "never had plans" to travel to the capital. Watch more on the convoy outside Gori »

Saakashvili said Russian forces "are encroaching upon the capital" of Georgia in violation of the cease-fire agreement. He said the Russians never intended to hold up their end of the truce.

"This is the kind of cease-fire that, I don't know, they had with Afghanistan, I guess, in 1979," Saakashvili said. "There is no cease-fire. [Russian forces] are moving around." Watch Saakashvili speak »

Interfax reported that a spokesman for the command of the Russian peacekeeping force said Russian soldiers who discovered an unguarded arms depot near Gori were emptying it as part of their goal of demilitarizing the area near South Ossetia.

Russian government spokesman Dmitry Peskov said Georgian government officials had fled Gori, and the Russian soldiers were providing food and water to residents.

There were reports of looting. Using a cell phone, one Gori resident said he was hiding in the basement of his house, and there was chaos in the city. Russian tanks were parked at a nearby Georgian military base, he said.

The Russian General Staff in Moscow accused the Georgians of not honoring the cease-fire, saying Georgian troops should return to their barracks.

"The response has not been adequate," Saakashvili said. "Not only those people who are committing all those atrocities are responsible, but those who don't react to that, I think they also share responsibility."

The six-point deal between the sides, negotiated by French President Nicolas Sarkozy, was meant to end the fighting over the breakaway regions of South Ossetia and Abkhazia. Watch more on push for peace »

However, Saakashvili, flanked by the leaders of Lithuania, Poland, Estonia and Latvia in a media briefing early Wednesday, said Russian tanks were attacking and "rampaging" through the Georgian town of Gori despite the cease-fire.

Journalists in Gori, the birthplace of former Soviet leader Joseph Stalin, said they had seen no Russian tanks. Watch more on battle-ravaged South Ossetia »

Reports of casualties on each side varied widely, and CNN could not confirm any of them.

Fighting has raged since Thursday when Georgia launched its crackdown on separatist fighters in autonomous South Ossetia, where most people have long supported independence or reunification with neighboring North Ossetia, a territory within Russia.

Russia sent its tanks into South Ossetia on Friday, saying it needed to protect Russian citizens living in the enclave, and quickly pushed back the Georgian forces. Russian forces also moved into Abkhazia, another breakaway Georgian region.